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Abstract:

Methods of leveling ink on substrates and apparatuses useful in printing
are provided. An exemplary embodiment of the methods includes irradiating
ink disposed on a surface of a porous substrate with radiation emitted by
at least one radiant energy source. The radiation heats the ink to at
least a viscosity threshold temperature of the ink to allow the ink to
flow laterally on the surface to produce leveling of the ink. The ink is
heated sufficiently rapidly that heat transfer from the ink to the
substrate is sufficiently small during the leveling that ink at the
substrate interface is cooled to a temperature below the viscosity
threshold temperature thereby preventing any significant ink permeation
into the substrate.

Claims:

1. A method of leveling ink on a substrate, the method comprising
irradiating ink disposed on a first surface of a porous substrate with
first radiation emitted by at least one first radiant energy source, the
first radiation heating the ink to at least a viscosity threshold
temperature of the ink to allow the ink to flow laterally on the first
surface to produce leveling of the ink, the ink being heated sufficiently
rapidly that heat transfer from the ink to the substrate is sufficiently
small during the leveling that ink at the substrate interface is cooled
to a temperature below the viscosity threshold temperature thereby
preventing any significant ink permeation into the substrate.

2. The method of claim 1, wherein the ink has a viscosity range of about
10.sup.1 to about 10.sup.6 cP over a temperature range.

3. The method of claim 2, wherein the temperature range is less than
about 40 Celsius degrees.

4. The method of claim 1, wherein the ink is a gel ink.

5. The method of claim 1, wherein substantially no curing of the ink is
produced by irradiating the ink with the first radiation emitted by the
at least one first radiant energy source.

6. The method of claim 1, wherein the ink is an ultraviolet light (UV)
curable ink.

7. The method of claim 6, further comprising irradiating ink on the first
surface of the substrate with UV radiation emitted by a second radiant
energy source to cross-link the ink subsequent to leveling of the ink.

8. The method of claim 1, wherein the first radiation has an emission
spectrum falling within the visible-infrared portion of the
electromagnetic spectrum.

9. The method of claim 1, wherein the first radiation has an emission
spectrum with emission peaks at more than one wavelength.

10. The method of claim 1, wherein the first radiation is monochromatic
light.

11. The method of claim 1, wherein the at least one first radiant energy
source comprises at least one lamp and a reflector positioned relative to
each lamp to reflect the first radiation onto the ink on the first
surface of the substrate.

12. The method of claim 1, further comprising: heating the ink to a
temperature greater than the viscosity threshold temperature; and
applying the heated ink to the first surface of the substrate with at
least one print head.

13. The method of claim 12, wherein the ink on the first surface of the
substrate is irradiated with the first radiation to level the ink
immediately after applying the ink to the first surface.

14. The method of claim 1, wherein the substrate is moved relative to the
at least one first radiant energy source while irradiating the ink with
the first radiation.

15. The method of claim 1, further comprising cooling the second surface
of the substrate while irradiating the ink with the first radiation.

16. The method of claim 1, wherein print-through (PT) of the ink in the
substrate has a value of less than about 0.04 as determined by the
equation: PT=show-through (ST)-OD(CP), in which ST is the optical density
of the second surface of the substrate, OD(CP) is the optical density of
the first surface of the substrate covered by a blank substrate of the
same material as the first substrate, and ST and OD(CP) are measured by a
densitometer.

17. A method of leveling ink on a substrate, the method comprising
irradiating a gel ink disposed on a surface of a substrate with first
radiation emitted by at least one first radiant energy source, the
surface being non-permeable with respect to the gel ink, the first
radiation rapidly heating the gel ink to at least a viscosity threshold
temperature of the gel ink to allow the gel ink to flow laterally on the
surface to produce leveling of the gel ink.

18. The method of claim 17, wherein substantially no curing of the gel
ink is produced by irradiating the gel ink with the first radiation
emitted by the at least one first radiant energy source.

19. The method of claim 17, further comprising irradiating the gel ink on
the surface of the substrate with UV radiation emitted by a second
radiant energy source to cross-link the gel ink subsequent to leveling of
the gel ink.

20. An apparatus useful in printing, comprising: a marking device for
applying ink to a first surface of a porous substrate, the ink having a
viscosity threshold temperature at which the ink has a viscosity midway
between a minimum value and a maximum value of the ink; and a leveling
device including at least one first radiant energy source which emits
first radiation onto ink applied to the first surface of the porous
substrate, the first radiation heating the ink to at least the viscosity
threshold temperature of the ink to allow the ink to flow laterally on
the first surface to produce leveling of the ink, the ink being heated
sufficiently rapidly that heat transfer from the ink to the substrate is
sufficiently small during the leveling that ink at the substrate
interface is cooled to a temperature below the viscosity threshold
temperature thereby preventing any significant ink permeation into the
substrate.

21. The apparatus of claim 20, wherein the first radiation emitted by the
at least one first radiant energy source has an emission spectrum falling
within the visible-infrared portion of the electromagnetic spectrum.

22. The apparatus of claim 20, wherein the at least one first radiant
energy source comprises at least one lamp and a reflector positioned
relative to each lamp to reflect the first radiation onto the ink
deposited on the first surface of the substrate.

23. The apparatus of claim 20, wherein the first radiation emitted by the
at least one radiant energy source has an emission spectrum with emission
peaks at more than one wavelength.

24. The apparatus of claim 20, wherein the first radiation emitted by the
at least one radiant energy source is monochromatic light.

25. The apparatus of claim 20, further comprising a transport device for
moving the substrate relative to the at least one first radiant energy
source while the ink is being irradiated with the first radiation.

26. The apparatus of claim 20, further comprising a device for cooling
the second surface of the substrate while the ink is being irradiated
with the first radiation by the at least one first radiant energy source.

27. The apparatus of claim 20, comprising a combined device including the
marking device and the leveling device, wherein the leveling device is
positioned to immediately emit the first radiation onto the ink after the
ink is applied to the first surface to level the ink.

28. The apparatus of claim 20, wherein: the first radiation emitted by
the at least one first radiant energy source produces substantially no
curing of the ink; and the apparatus further comprises a second radiant
energy source for irradiating ink on the first surface of the substrate
with UV radiation to cross-link the ink subsequent to leveling of the
ink.

Description:

RELATED APPLICATIONS

[0001] This application is related to the application entitled "METHODS OF
LEVELING INK ON SUBSTRATES USING FLASH HEATING AND APPARATUSES USEFUL IN
PRINTING," Attorney Docket No. 056-0203, which is filed on the same date
as the present application.

BACKGROUND

[0002] In printing processes, marking material is applied onto substrates
to form images. In some processes, the printed images can exhibit
microbanding and print-through on the substrates.

[0003] It would be desirable to provide methods of leveling ink on
substrates and apparatuses useful in printing that can produce
high-quality printed images on different types of substrates.

SUMMARY

[0004] Methods of leveling ink on substrates and apparatuses useful in
printing are provided. An exemplary embodiment of the methods of leveling
ink on a substrate comprises irradiating ink disposed on a first surface
of a porous substrate with first radiation emitted by at least one first
radiant energy source. The first radiation heats the ink to at least a
viscosity threshold temperature of the ink to allow the ink to flow
laterally on the first surface to produce leveling of the ink. The ink is
heated sufficiently rapidly that heat transfer from the ink to the
substrate is sufficiently small during the leveling that ink at the
substrate interface is cooled to a temperature below the viscosity
threshold temperature thereby preventing any significant ink permeation
into the substrate.

DRAWINGS

[0005]FIG. 1 depicts a curve illustrating the relationship between
marking material viscosity and temperature for an exemplary marking
material.

[0006]FIG. 2 depicts an exemplary embodiment of an apparatus useful for
printing including a marking device, leveling device and optional curing
device.

[0007]FIG. 3 depicts an exemplary embodiment of a radiant energy source
of the leveling device.

[0010] FIGS. 6A to 6F show pictures, top side left to right, of
600×600 dpi patches (modified with every seventh line blank) and
600×300 dpi patches each with a width of 0.5 in. The patches were
printed with a standard black UV gel ink containing 7.5 wt % gel and 5 wt
% wax on 4200 paper. FIG. 6A shows as-printed patches. FIGS. 6B to 6F
show patches following leveling using a tungsten lamp (rated power of
1200 W at rated lamp voltage of 144 V, actual lamp voltage of 208 V,
actual power of 2114 W) for paper transport speeds of 1000 mm/s, 750
mm/s, 500 mm/s, 250 mm/s and 125 mm/s, respectively. The pictures are
viewed from the top side left to right (left half of FIGS. 6A to 6F) and
bottom side right to left (right half of FIGS. 6A to 6F) of the paper.

[0012] FIGS. 8A to 8F show pictures, top side right to left, of
600×600 dpi patches, 600×600 dpi patches modified with every
seventh line blank, 600×150 dpi patches, and 150×150 dpi
patches, each having a width of 0.5 in. The patches were printed with a
standard cyan UV gel ink formulation containing 7.5 wt % gel and 5 wt %
wax on 4200 paper. FIG. 8A shows as-printed patches. FIGS. 8B to 8F show
patches following leveling using a tungsten lamp (rated power of 500 W at
rated lamp voltage of 120 V, actual lamp voltage of 208 V, actual power
of 1166 W) for paper transport speeds of 1000 mm/s, 750 mm/s, 500 mm/s,
250 mm/s and 125 mm/s, respectively. The pictures are viewed from the top
side right to left (left half of FIGS. 8A to 8F) and bottom side left to
right (right half of FIGS. 8A to 8F) of the paper.

[0014] FIGS. 10A to 10F show pictures, top side right to left, of
600×600 dpi patches, 600×600 dpi patches modified with every
seventh line blank, 600×150 dpi patches, and 150×150 dpi
patches, each having a width of 0.5 in.

[0015] The patches were printed with a cyan UV gel ink containing 10 wt %
gel and 10 wt % wax on 4200 paper. FIG. 10A shows as-printed patches.
FIGS. 10B to 10F show patches following leveling using a tungsten lamp
(rated power of 500 W at rated lamp voltage of 120 V, actual lamp voltage
of 208 V, actual power of 1166 W) for paper transport speeds of 1000
mm/s, 750 mm/s, 500 mm/s, 250 mm/s and 125 mm/s, respectively. The
pictures are viewed from the top side right to left (left half of FIGS.
10A to 10F) and bottom side left to right (right half of FIGS. 10A to
10F) of the paper.

[0017] The disclosed embodiments include methods of leveling ink on
substrates. An exemplary embodiment of the methods comprises irradiating
ink disposed on a first surface of a porous substrate with first
radiation emitted by at least one first radiant energy source. The first
radiation heats the ink to at least a viscosity threshold temperature of
the ink to allow the ink to flow laterally on the first surface to
produce leveling of the ink. The ink is heated sufficiently rapidly that
heat transfer from the ink to the substrate is sufficiently small during
the leveling that ink at the substrate interface is cooled to a
temperature below the viscosity threshold temperature thereby preventing
any significant ink permeation into the substrate.

[0018] Another exemplary embodiment of the methods of leveling ink on
substrates comprises irradiating a gel ink disposed on a surface of a
substrate with first radiation emitted by at least one first radiant
energy source. The surface is non-permeable with respect to the gel ink.
The first radiation rapidly heats the gel ink to at least a viscosity
threshold temperature of the gel ink to allow the gel ink to flow
laterally on the surface to produce leveling of the gel ink.

[0019] The disclosed embodiments further include apparatuses useful in
printing. An exemplary embodiment of the apparatuses comprises a marking
device for applying ink to a first surface of a porous substrate, the ink
having a viscosity threshold temperature at which the ink has a viscosity
midway between a minimum value and a maximum value of the ink; and a
leveling device including at least one first radiant energy source which
emits first radiation onto ink applied to the first surface of the porous
substrate. The first radiation heats the ink to at least the viscosity
threshold temperature of the ink to allow the ink to flow laterally on
the first surface to produce leveling of the ink. The ink is heated
sufficiently rapidly that heat transfer from the ink to the substrate is
sufficiently small during the leveling that ink at the substrate
interface is cooled to a temperature below the viscosity threshold
temperature thereby preventing any significant ink permeation into the
substrate.

[0020] Ultraviolet light (UV)-curable inks can be used in printing
processes to form images on substrates. UV-curable inks are applied to a
surface of a substrate and then exposed to UV light to cure the ink and
fix images onto the surface. It has been noted that low-viscosity,
UV-curable inks display an unacceptably-high degree of print-through when
applied on plain paper substrates, which are porous. Print-through is a
measure of ink permeation in the thickness direction of the substrates.
Print-through makes low-viscosity, UV-curable inks unsatisfactory for
printing applications with plain paper substrates.

[0021] UV-curable gel inks ("UV gel inks") are another type of marking
material that can be used to form images on substrates. These inks offer
desirable properties including higher viscosities than conventional,
low-viscosity, UV-curable inks. UV gel inks are heated to abruptly reduce
their viscosity and then applied to substrates. These inks freeze upon
contact with the cooler substrates. It has been noted that freezing of UV
gel inks upon initial impingement onto substrates, such as paper, and ink
drop misdirection can result in micro-banding of images formed on the
substrates.

[0022] UV-curable inks applied to substrates can be leveled by applying
pressure to the inks as disclosed in U.S. patent application Ser. No.
12/256,670 to Roof et al., filed on Oct. 23, 2008 and entitled "Method
and Apparatus for Fixing a Radiation-Curable Gel-Ink Image on a
Substrate"; U.S. patent application Ser. No. 12/256,684 to Roof et al.,
filed on Oct. 23, 2008 and entitled "Dual-Web Apparatus for Fixing a
Radiation-Curable Gel-Ink Image on a Substrate" and U.S. patent
application Ser. No. 12/256,690 to Roof et al., filed on Oct. 23, 2008
and entitled "Apparatus for Fixing a Radiation-Curable Gel-Ink Image on a
Substrate," each of which is incorporated herein by reference in its
entirety.

[0023] Images formed on substrates using UV gel inks can be leveled
without physical contact with the images using an IR-VIS
(infrared-visible radiation) radiant energy source. It has been noted
that extended heating of UV gel inks using such sources can produce
print-through on porous, plain paper substrates due to the amount of
energy that is transferred to the substrates during the extended heating
period, and the subsequent penetration of the ink through the warm paper.

[0024] In view of these observations regarding UV gel inks, as well as
other types of inks, methods of leveling ink on substrates and
apparatuses useful in printing that can be used to perform the methods
are provided. Embodiments of the methods and apparatuses can level
different types of inks on substrates. The inks used to form images on
substrates can be any suitable ink composition that thermally quenches
into a sufficiently-rigid state and has a sufficiently-sharp melting
transition at an elevated temperature relative to the substrate
temperature. Exemplary inks can exhibit a viscosity range of about
101 to about 106 cP over a temperature range of less than about
40 Celsius degrees, less than about 30 Celsius degrees, less than about
20 Celsius degrees, or less than about 10 Celsius degrees, for example.

[0025] For example, gel inks can be leveled on substrates in embodiments
of the methods and apparatuses. FIG. 1 depicts a curve illustrating the
viscosity as a function of temperature for a typical gel ink that has
properties compatible with exemplary embodiments of the disclosed methods
of leveling ink on substrates. As shown, the viscosity profile for the
gel ink has a sharp threshold and the ink transitions from being
relatively viscous (having a viscosity of, e.g., on the order or greater
than about 106 cP) and unable to flow easily, to being relatively
non-viscous (having a viscosity of, e.g., on the order of less than about
101 cP) and able to flow easily over a relatively narrow temperature
range where. Such gel inks can exhibit a large change in viscosity over a
small temperature range of less than about 40 Celsius degrees, less than
about 30 Celsius degrees, less than about 20 Celsius degrees, or less
than about 10 Celsius degrees, for example. Such gel inks thermally
quench into a sufficiently-rigid state and have a sufficiently-sharp
melting transition at an elevated temperature relative to the substrate
temperature to be compatible with exemplary embodiments of the disclosed
methods of leveling inks on substrates.

[0026] Exemplary inks having properties as depicted in FIG. 1 and which
can be used to form images on substrates in embodiments of the disclosed
methods and apparatuses are described in U.S. Patent Application
Publication No. 2007/0120919, which discloses a phase change ink
comprising a colorant, an initiator, and an ink vehicle; in U.S. Patent
Application Publication No. 2007/0123606, which discloses a phase change
ink comprising a colorant, an initiator, and a phase change ink carrier;
and in U.S. Pat. No. 7,559,639, which discloses a radiation curable ink
comprising a curable monomer that is liquid at 25° C., curable wax
and colorant that together form a radiation curable ink, each of which is
incorporated herein by reference in its entirety.

[0027] In the curve shown in FIG. 1, there is a viscosity threshold
temperature T0, which is defined as the temperature at which the
viscosity of the ink is midway between its minimum and maximum values. At
T0, the viscosity of the ink is sufficiently low such that it can
flow easily. T0 can typically range from about 55° C. to
about 65° C. for exemplary gel inks. In exemplary embodiments, the
ink is heated to at least the viscosity threshold temperature to allow
the ink to flow sufficiently under the influence of surface/interfacial
tension and interfacial capillary forces on a surface of a substrate.

[0028] Embodiments of the methods and apparatuses can level images formed
on substrates to mitigate micro-banding of the images without physical
contact with the images during the leveling. Embodiments of the methods
and apparatuses can level inks on porous substrates with minimal
print-through of the inks. Such porous substrates have open porosity
extending from a front surface, on which the inks are deposited, toward
an opposite back surface, on which inks can also be deposited. The open
porosity can extend partially or completely through the thickness
dimension of the substrate defined by the front and back surfaces. The
pores are permeable to the ink. Show-through (ST) is defined as the back
surface optical density. If OD(CP) is defined as the optical density (OD)
of the front surface of a substrate covered by a blank sheet of a paper
substrate, then print-through (PT) is defined as: PT=ST-OD(CP). In
embodiments, the PT value is less than about 0.04, such as less than
about 0.035, less than about 0.03, or less than about 0.025.

[0029] The methods and apparatuses can also be used to level inks, such as
gel inks, and the like, on substrates other than plain paper, such as
coated paper, plastic and metal films and laminates. These substrates can
include a surface on which inks are deposited that is non-permeable with
respect to the ink. The substrates can be composed of heat-sensitive
materials, such as heat-sensitive plastics. Embodiments of the
apparatuses can be used in xerography, lithography and flexography.

[0030] Embodiments of the apparatuses include at least one radiant energy
source that emits radiation to heat inks on substrates. The emitted
radiation produces a short-duration exposure over a small distance of the
substrate. The radiation exposure supplies sufficient thermal energy to
the inks to heat them to a point to reduce their viscosity to enable the
inks to level by surface-tension driven lateral reflow on substrate
surfaces. This lateral reflow mitigates micro-banding of images formed by
the inks.

[0031] In embodiments, the radiation exposure desirably is sufficiently
high and sufficiently brief to produce only minimal heat transfer from
the ink to the substrate. This heat transfer desirably is insufficient to
heat the substrate in contact with the ink to a temperature above the ink
melting point. The radiation exposure can be effective to minimize
print-through of gel inks, and the like, on porous substrates, such as
plain paper.

[0032] Regarding the heating time of the inks on substrates, when the
radiant energy source emits radiation at a fixed power level, a shorter
pulse deposits less energy and heats inks less. The amount of radiant
energy deposited can also be kept constant by raising the power level. In
such embodiments, a shorter pulse at the higher power level results in a
higher rate of temperature rise of inks. By optimizing the absorption of
the radiant energy in the inks and using a desirably strong radiant
energy source, the inks can be heated in a desirably short time,
tRAD.

[0033] When an ink on a surface of a porous substrate is at a particular
temperature, the ink viscosity and surface tensions allow lateral reflow
on the surface to reduce the surface area of the ink. The amount of time
to achieve this lateral reflow of the ink is tL-R. Similarly,
capillary forces within the pores of the substrate lead to permeation
into the substrate. The amount of time for the ink to permeate a given
distance in such pores is tPERM. Also, heat absorbed in the ink
transfers by thermal conduction into the cooler substrate, heating the
near-surface region of the substrate most and being conducted eventually
to the opposite face of the substrate. There is a characteristic time,
tDIFF, for such thermal diffusion to occur in substrates. The value
of tDIFF depends on factors including the heat capacity and thermal
diffusivity of the substrate, as well as temperature gradients.

[0034] In embodiments of the leveling process, the following relationships
between these time values are desirable: tRAD is comparable with,
and shorter than tL-R and tPERM; tPERM is longer than
tL-R; and tL-R is much shorter than tDIFF. These
relationships can be written as follows:
tRAD≦tL-R<tPERM<<tDIFF. When
tDIFF is sufficiently long, even if tPERM is short, the thermal
gradient in the substrate will be sufficiently high and the ink will be
quenched near the top surface of the substrate and mainly reflow
laterally along that surface.

[0035]FIG. 2 depicts an exemplary embodiment of an apparatus 100 useful
in printing. The apparatus 100 includes a marking device 110 for
depositing ink onto substrates, and a leveling device 120 for irradiating
the as-deposited ink with radiation of a selected spectrum to level the
ink. The illustrated apparatus 100 also includes an optional UV curing
device 130 for radiating as-leveled, UV-curable inks with UV radiation to
cross-link the inks and provide robustness, when such inks are optionally
used to form images on substrates.

[0036]FIG. 2 shows a substrate 140 supported on a transport device 150.
The transport device 150 can be a belt, or the like. Other types of
devices, such as rollers, can also be used to transport the substrate
140. An as-applied layer of ink 144 is shown on the top surface 142 of
the substrate 140. The transport device 150 transports the substrate 140
in the process direction, A, past the marking device 110, leveling device
120 and the optional curing device 130 to produce images on the substrate
140. The leveling device 120 can typically be spaced from the marking
device 110 by a distance of about 10 cm to about 50 cm along the process
direction A. For a substrate 140 in the form of a continuous web, a
stationary support device can be used in place of the transport device
150 and the web may be pulled over the support device configured to hold
the web at a fixed distance from the marking device 110, leveling device
120 and optional curing device 130.

[0037] The marking device 110 can include one or more print heads (not
shown). For example, the print heads can be heated piezo print heads.
Typically, the marking device 110 includes a series of print heads. The
print heads can typically be arranged in multiple, staggered rows in the
marking device 110. The print heads can be constructed of stainless
steel, or the like. The print heads can provide a modular, scalable array
for making prints using different sizes of substrates. The print heads
can use cyan, magenta, yellow and black inks, to allow inks of different
colors to be printed atop each other.

[0038] The print heads can heat the ink to a sufficiently-high temperature
to reduce the ink viscosity to the desired viscosity for jetting from the
nozzles. For example, gel inks can be heated to a temperature above the
viscosity threshold temperature. The hot ink is jetted as droplets from
the nozzles of the print heads onto substrates being transported past the
marking device 110. The print heads can produce the desired drop size and
enable high-speed production.

[0039] Gel inks, such as UV gel inks, can be used in the print heads of
the marking device 110. In other embodiments, other types of inks having
suitable properties, such as wax inks, and the like, can be used in the
marking device 110 to form images. Such inks can exhibit a large change
in viscosity over a small change in temperature during cooling or
heating. UV gel inks can typically be heated to a temperature of at least
about 80° C. in the print heads to develop the desired viscosity
for jetting. UV gel inks can typically exhibit a large increase in
viscosity when they are cooled from the jetting temperature by about
10° C., e.g., from about 80° C. to about 70° C. When
the ink impinges on a substrate, such as plain paper, heat is transferred
from the ink to the cooler substrate. The as-deposited ink rapidly cools
and develops a gel consistency on the substrate. Due to the rapid
cooling, the ink does not have sufficient time to reflow laterally, or
level, on the substrate. Consequently, images formed on the substrates
with the inks can display microbanding.

[0040] Positive pressure pumps with computer controlled needle valves,
such as a Smart Pump® 20, available from nScrypt, Inc. of Orlando,
Fla., can be used to eject inks. These pumps can eject very small volumes
down to picoliters, at very high viscosities, such as viscosities above
106 cP. Such pumps can be used to deposit gel inks at room
temperature onto substrates. The deposited gel inks can then be leveled
by embodiments of the apparatuses and methods described herein.

[0041] The leveling device 120 includes at least one radiant energy source
that emits radiant energy onto the ink 144. The radiant energy can have
an emission spectrum falling within the visible-infrared portion of the
electromagnetic spectrum. In embodiments, the radiant energy source can
be, e.g., a broad-band, IR-VIS (infrared-visible radiation) radiant
energy source with an emission spectrum that covers the visible range
(˜400 nm to 700 nm) and extends into the infrared range (>700
nm).

[0042]FIG. 3 shows a substrate 240 positioned under an exemplary radiant
energy source 224 of a leveling device. The substrate 240 is moved
relative to the radiant energy source 224 on a transport device 250. The
transport device 250 is movable in the process direction A to transport
the substrate 240 past the marking device (not shown) and leveling
device. An optional curing device (not shown) can also be used in some
embodiments. The substrate 240 is typically oriented relative to the
leveling device with the length dimension of the substrate extending
along the process direction A. The radiant energy source 224 can
typically be spaced from about 2 cm to about 5 cm from the surface of the
substrate and from about 10 cm to about 50 cm downstream from the print
heads along the process direction A. In embodiments, the substrate 240
can be a continuous web. For a continuous web, a stationary support
device can be used in place of the transport device 250 and the web may
be pulled over the support device to hold the web at a fixed distance
from the marking device.

[0043] The substrate 240 includes a top surface 242. A layer of ink 244 is
shown on the top surface 242. In the illustrated embodiment, the radiant
energy source 224 is a lamp. A curved reflector 226 is configured to
focus radiant energy emitted by the lamp onto the ink 244, to produce an
exposure zone with a small focal width, along the length dimension of the
substrate 240. The lamp produces an emission spectrum suitable for
irradiating selected ink compositions. For example, the lamp can be a
tungsten halogen lamp, or the like. In such lamps, the color temperature
(i.e., the wavelength of the emission spectrum peak) can be adjusted to
increase the amount of overlap between the lamp emission spectrum and the
absorption spectrum of the ink. The leveling device can include a filter
to transmit only a selected portion of the IR-VIS spectrum emitted by the
radiant energy source.

[0044] In other embodiments, the leveling device can include at least one
radiant energy source that emits radiation with emission peaks at several
different wavelengths, such as a mercury discharge lamp, or the like.

[0045] In other embodiments, the leveling device can include at least one
monochromatic radiant energy source that emits radiant energy at a single
wavelength. For example, the radiant energy source can be a laser, such
as a semiconductor diode laser or a laser array. A light-emitting diode
array, or the like, can also be used.

[0046] The different radiant energy sources that can be used in the
leveling device can achieve an exposure zone focal width ranging from
about 0.5 mm to about 10 mm, for example. The leveling device can include
a radiant energy guide, or the like, to direct radiant energy emitted by
the radiant energy source over a small region of the substrate to reduce
the ink surface that is irradiated.

[0047] In embodiments, the radiant energy source is stationary and the
substrate is moved past the radiant energy source to radiate the
substrate. At a given transport speed of the substrate relative to the
leveling device, reducing the focal width of the radiant energy source
reduces the exposure time of ink on the substrate. For single radiant
energy sources, such as a tungsten filament extended across the width
dimension of the substrate perpendicular to the process direction, the
radiant energy source can be turned ON throughout the leveling process to
allow the entire substrate surface to be irradiated as the substrate is
moved past the radiant energy source.

[0048] In other embodiments, the radiant energy source can be movable to
allow radiation to be scanned over the substrate. For example, the
radiant energy source can be a laser extending continuously across the
width of the substrate, or a laser including laser bars arrayed in
segments along the width dimension of the substrate. Lasers can be
focused to scan a narrow line having a focal width of, e.g., less than
about 1 mm in the process direction on the substrate. For such radiant
energy sources, the radiation can be emitted only to irradiate regions of
the substrate surface where ink is present to limit heating of the
substrate and to limit unnecessary power consumption.

[0049] The base supporting the substrate can be a cooled heat sink to
transfer heat away from the substrate during irradiation of the ink at
the leveling device to control the ink and substrate temperatures during
the leveling process, to minimize print-through.

[0050] In other embodiments, the substrate may not be supported on a heat
sink when sufficient lateral reflow of ink on the substrate can be
achieved without concern that the substrate may reach a sufficiently-high
temperature during radiation of the ink to result in more than a minimal
amount of vertical transport of the ink in porous substrates. In
embodiments, some amount of vertical transport of the ink is desired to
provide sufficient fixing of ink to porous substrates. In non-porous
substrates, such as non-porous plastics and metals, chemical bonding of
the ink to the substrate surface, and micro-porosity at the substrate
surface, can provide sufficient fixing of the ink to the surface.

[0051] In the apparatus 100 shown in FIG. 2, the substrate 140 moves in
the process direction A at a selected speed relative to the stationary
leveling device 120. The radiant energy source of the leveling device 120
irradiates the ink 144 as the substrate 140 is moved relative to the
radiant energy source. The radiant energy source can emit radiation over
a distance in the process direction A of only about 0.5 to about 10 mm,
depending on the particular source used. The substrate 140 can typically
be moved at a speed up to about 1 m/s relative to the radiant energy
source. The ink 144 on the substrate 140 is irradiated for only a short
amount of time as the substrate 140 is moved relative to the radiant
energy source. For example, a radiant energy source that emits focused
radiation over a distance of about 10 mm can provide an exposure time of
the ink of about 10 ms for a substrate speed of about 1 m/s.
More-tightly-focused sources can be used to enable shorter exposure times
and thermal transfer times of inks. Increasing the transport speed of the
substrate can be used to reduce the exposure time of the ink 144 on the
substrate 140.

[0052] In the apparatus 100, the radiation emitted by the radiant energy
source onto the ink 144 is effective to heat the ink and lower the ink
viscosity sufficiently to allow lateral reflow, or thermal reflow
leveling, of the ink on the top surface 142 of the substrate 140. The ink
can be partially melted or fully melted by the radiant energy, with full
melting producing greater reflow coverage and more desirable leveling.
The ink can be heated sufficiently rapidly by the radiant energy source
that heat transfer from the ink to the substrate 140 is sufficiently
small during the leveling that ink at the substrate interface is cooled
to a temperature below the viscosity threshold temperature thereby
preventing any significant ink permeation into the substrate 140. The
"substrate interface" is defined as where the ink contacts the substrate,
which may be at the top surface 142, or below the top surface 142.
Penetration of the ink 144 into the substrate 140 resulting from heating
can be limited to a maximum depth of, e.g., less than about 20 μm,
less than about 10 μm, less than about 5 μm, less than about 4
μm, less than about 3 μm, or less than about 2 μm. Consequently,
print-through of porous substrates, such as plain paper, by vertical ink
flow can be substantially eliminated. The lateral reflow of the ink 144
improves optical density by mitigating micro-banding of the ink 144 on
the substrate 140.

[0053] Different inks that can be used in embodiments of the methods and
apparatuses can have different viscosities and surface tensions at the
leveling target temperature. Leveling process parameters including dwell
time and the irradiation power and emission spectrum of the radiant
energy source can be selected to be compatible with the properties of the
inks used in the methods and apparatuses, to produce desirable reflow and
leveling of the inks driven by surface tension and capillary forces.

[0054] FIG. 4 depicts an exemplary embodiment of a device 360 that
provides both marking and leveling functions. As shown, the device 360
includes a marking section 310 and a leveling section 320 positioned
downstream about 0.5 cm to about 5 cm from the marking section 310 along
the process direction A. A substrate 340 is shown supported on a
transport device 350 to move the substrate 340 along the process
direction A. The marking section 310 can include a single print head (not
shown), for example. The leveling device 320 includes at least one
radiant energy source (not shown). The radiant energy source can be a
broad band IR-VIS radiant energy source, such as a tungsten lamp, or the
like; a radiant energy source that can emit at more than one wavelength;
or a monochromatic radiant energy source. During operation, hot ink drops
312 are jetted from the print head, or ambient-temperature ink drops are
ejected from a positive pressure pump, onto the substrate 340, and then
immediately irradiated with radiation 322 from the radiant energy source
to maintain/bring the hot jetted ink at/to leveling temperature for a
sufficient amount of time to achieve the desired reflow. In embodiments,
the substrate 340 can be a continuous web. For a continuous web, a
stationary support device can be used in place of the transport device
350 and the web may be pulled over the support device constructed to hold
the web at a fixed distance from the marking device 310, leveling device
320 and the optional curing device.

[0055] The immediate irradiation of as-deposited ink on the substrate 340
can at least substantially eliminate the need to melt solidified ink
(using an additional amount of thermal energy) on the substrate 340 in
order to have thermal reflow leveling of completely-liquid ink.
Irradiating the ink immediately after deposition with the
marking/leveling device 360 can increase the total amount of time that
the ink remains at temperatures above the low-viscosity transition due to
the as-deposited ink either having a smaller temperature drop before
being reheated to the leveling temperature, or being maintained at a
substantially-constant temperature that is sufficient for leveling. The
combined marking/leveling device 360 can reduce the total amount of
energy that is sufficient to achieve the desired leveling, the total
time, and the total process waterfront needed for marking and leveling.

[0056] In cases where the heating power of the radiant energy source may
be limited, the combined marking/leveling device can enable a higher
process speed to be used because a smaller amount of thermal energy from
the radiant energy source can be sufficient to achieve the desired
leveling, as thermal energy in the as-applied ink is used for the
leveling. The same amount of power emitted by the radiant energy source
can heat the ink to a higher temperature at a fixed process speed. A
higher process speed can be used with the ink maintained at the desired
leveling temperature.

[0057] Embodiments of the apparatuses including a combined
marking/leveling device can use a radiant energy source for each print
head and each stage of marking, in contrast to performing leveling after
ink has been deposited on substrates from all print heads of marking
devices including multiple print heads. In apparatuses including a
combined marking/leveling device, the amount of radiation emitted from
each radiant energy source can be set based on the amount of ink
deposited at each associated print head, which allows close control of
the amount and duration of each exposure.

[0058] Black inks have a broad absorption band that extends across a
substantial portion of the emission spectrum of IR-VIS lamps. For other
ink colors, such as cyan, which have a narrower absorption band than
black inks, to provide a significant effect with respect to preventing
print-through on porous substrates, the color temperature of the IR-VIS
lamp can be raised relative to the temperature used for leveling black
inks, and the ink formulations can be changed to contain a higher gel and
wax content.

[0059] Gel ink formulations can be tuned by adding one or more IR
absorbers, to increase the amount of overlap between the lamp emission
spectrum and the absorption spectrum of the ink.

[0061] Carbon black ink has a high absorbance over the entire visible and
near IR region. As shown in FIG. 5, in general the absorbance of cyan ink
is predominantly in the red region of the visible spectrum. To achieve
higher absorbance of such cyan inks, the color temperature of the radiant
energy source (e.g., tungsten halogen lamp) can be increased and/or an IR
absorber can be added to the cyan ink. FIG. 5 shows poor overlap of the
emission spectrum of a tungsten lamp operated at a temperature of 2500K
with a cyan pigment, or with an IR absorbing dye. The overlap is
considerably better when the tungsten lamp is operated at a higher
temperature of 3000K.

[0062] In other embodiments, the radiant energy source(s) of the leveling
device can be a monochromatic source, such as a scanning laser focused to
scan a narrow line across substrates in the cross-process direction. To
level cyan, magenta or yellow inks containing an IR absorbing pigment or
dye, the laser can be selected to emit radiation at a wavelength of,
e.g., 1.06 μm or 0.9 μm (GaAs) depending on the absorption spectrum
of the IR pigment or dye. The radiant energy source can also be an arc
lamp, such as a deuterium lamp, which in addition to an output of
leveling radiation in the visible region of the spectrum (400-700 nm),
also has significant output of curing radiation in the UV region of the
spectrum (200-400 nm).

EXAMPLES

Example 1

[0063] Black ink was deposited on plain paper and then irradiated to level
the ink. In Example 1, a tungsten halogen lamp with an elliptical
reflector (FIG. 3) was used to produce an approximately 10 mm focal width
exposure zone and to irradiate the ink deposited on the paper. The
tungsten halogen lamp was a Model No. GE QH 1200W HT 144V from General
Electric Co. The lamp had a rated power of 1200 W with a color
temperature of 2450K when driven at the rated lamp voltage of 144 V. The
lamp was operated at an actual lamp voltage of 208 V and actual power of
2114 W (423 W/in) with a color temperature of about 2812K.

[0064] The lamp generally irradiated beyond the edges of the paper. The
paper substrates were supported on a water-cooled cold shoe maintained at
a temperature of about 10° C. The cold shoe dissipated heat
transferred to the substrate during the irradiation to cool the substrate
and hinder ink penetration through the paper. To provide effective
thermal transfer to the cold shoe, the paper was held in contact with the
top surface of the cold shoe using 3M® Spray Mount® Artist's
Adhesive, available from 3M of Saint Paul, Minn. This thermal contact was
maintained during the entire process of depositing ink on the paper,
off-line leveling and off-line UV curing.

[0065] A series of images was printed onto Xerox 4200 paper using a
standard black ink formulation (BK30557-31) containing 7.5 wt % gel and 5
wt % wax with a modified 600×600 dpi patch (every seventh line
blank) beside a 600×300 dpi patch. To investigate the ability of
the focused IR lamp to produce desirable lateral leveling without
significant paper heating and associated vertical ink penetration and
print through, the printed patches were passed under the lamp at a series
of decreasing transport speeds, ranging from 1 m/s down to 125 mm/s. The
top (front) surface optical density (OD) of the 600×600 dpi patches
was used as a quantitative measure of the lateral ink spreading.
Print-through was used as a quantitative measure of vertical ink
penetration from the top surface through the paper. Show-through (ST) was
defined as the optical density of the back surface of the paper. Defining
OD(CP) as the optical density of the top surface of the paper covered
with a blank sheet of the paper substrate, print-through (PT) was defined
as: PT=ST-OD(CP). OD, OD(CP) and ST were measured with a Gretag Macbeth
model RD-918 densitometer. A print-through value of less than 0.025 was
not visually objectionable and was considered to be acceptable. A
print-through value of 0.025 was visually objectionable and considered to
be unacceptable.

[0066] Pictures of the printed patches taken from the top and the bottom
sides of the paper substrates are shown in FIGS. 6A to 6F. FIG. 6A shows
as-printed patches and FIGS. 6B to 6F show patches following leveling for
paper transport speeds of 1000 mm/s, 750 mm/s, 500 mm/s, 250 m/s and 125
mm/s, respectively.

[0067]FIG. 7 illustrates curves showing the optical density and the
corresponding print-through for the as-leveled 600×600 dpi patches
depicted in FIGS. 6B to 6F. The as-printed optical density and
print-through for the patches depicted in FIG. 6A are also shown for
comparison.

[0068] As shown, the optical density of the 600×600 dpi patch
leveled at a transport speed (process speed) of 1 m/s increases over that
of the as-printed substrate due to lateral ink spreading. The desired
leveling is achieved. The optical density of the substrate leveled at a
transport speed of 750 mm/s also increases slightly with respect to the
substrate leveled at 1 m/s. The desired leveling is achieved. The optical
density of the substrate leveled at a transport speed of 500 mm/s
decreases to the optical density of the as-printed substrate due to
print-through starting to occur. Further reduction in the transport
speed/increase in exposure, at speeds of 250 mm/s and 125 mm/s, results
in higher print-through and the optical density decreasing to below that
of the as-printed substrate.

[0069] The test results as plotted in FIG. 7 and as viewed in FIGS. 6A to
6F show that the focused IR-VIS lamp at a color temperature of about
2800K achieves good leveling of black ink without unacceptable
print-through, PT 0.025, over a process window in the region of at least
about 750 mm/s to 1000 mm/s. This is consistent with the visual
appearance of the back sides of the stress case 600×600 dpi images
in FIGS. 6B and 6C, which are not judged to be objectionable, and are
acceptable. For throughput speeds of 500 mm/s or slower, as seen in FIGS.
6D to 6F, the print-through is unacceptable, PT 0.025, and it increases
with reducing speed or increasing dwell time in the lamp exposure zone.

Example 2

[0070] A standard cyan ink formulation (BK30461-68A) containing 7.5 wt %
gel and 5 wt % wax was used. To increase the overlap of the emission
spectrum of the radiant energy source with respect to the absorbance
spectrum of the cyan ink, a different lamp was used to increase the color
temperature achievable with a voltage of 208V. The lamp was a model
500T3/CL available from Research Inc., of Eden Prairie, Minnesota. The
lamp has a rated power of 500W with a color temperature of 2500K when
driven with a rated voltage of 120V. The lamp was driven at an actual
voltage of 208V with an actual power of 1166 W and an actual color
temperature of 3073K.

[0071] A series of images was printed onto Xerox 4200 paper using the
standard cyan UV gel ink formulation. FIGS. 8A to 8F show pictures, top
side right to left (left half of FIGS. 8A to 8F), and bottom side left to
right (right half of FIGS. 8A to 8F) of 600×600 dpi patches,
600×600 dpi patches modified with every seventh line blank,
600×150 dpi patches, and 150×150 dpi patches. The printed
cyan patches were transported under the lamp operating at the color
temperature of 3073K at speeds of 1000 mm/s, 750 mm/s, 500 mm/s, 250 mm/s
and 125 mm/s. The optical density of the unmodified 600×600 dpi
patches was used as a measure of the lateral ink spreading, and
print-through was used as a measure of ink penetration through the paper.

[0072] Pictures of the printed patches from the top and bottom sides are
shown in FIGS. 8A to 8E. FIG. 8A shows as-printed patches. FIGS. 8B to 8F
show patches following leveling for paper transport speeds of 1000 mm/s,
750 mm/s, 500 mm/s, 250 mm/s and 125 mm/s, respectively.

[0073] FIG. 9 illustrates curves showing the optical density and the
corresponding print-through for the as-leveled 600×600 dpi patches
depicted in FIGS. 8B to 8F. The as-printed optical density and
print-through for the patches depicted in FIG. 8A are also shown for
comparison.

[0074] In general, all samples exhibit undesirably-high print-through as
judged by the visual appearance of the back side images in FIG. 8. For
all process conditions, the appearance of the back side of the
600×600 dpi areas is visually objectionable and unacceptable. This
is consistent with the measured print-through in FIG. 9, where PT 0.025
for all images. Print-through also increases as throughput speed
decreases and dwell time increases. Although the standard cyan ink
absorbs more energy at the higher color temperature exposure, there is no
window of operation at the substrate transport speeds used where the cyan
ink is leveled with acceptable print-through.

Example 3

[0075] Example 2 was repeated using the same lamp illumination conditions,
but with a high-gel (10 wt %) and high-wax (10 wt %) cyan ink formulation
(JBJF30554-15) to provide more process latitude for leveling ink and
acceptable print-through.

[0076] A series of images were printed onto 4200 paper using the high-gel
and high-wax cyan ink. FIGS. 10A to 10F show pictures, top side right to
left and bottom side also right to left, of 600×600 dpi patches,
600×600 dpi patches modified with every seventh line blank,
600×150 dpi patches, and 150×150 dpi patches. The printed
cyan patches were transported under the lamp operating at the color
temperature of 3073K at speeds of 1000 mm/s, 750 mm/s, 500 mm/s, 250 mm/s
and 125 mm/s. The optical density of the unmodified 600×600 dpi
patches was used as a measure of the lateral ink spreading, and
print-through was used as a measure of ink penetration through the paper.

[0077] FIG. 11 illustrates curves showing the optical density and the
corresponding print-through for the as-leveled 600×600 dpi patches
depicted in FIGS. 10B to 10F. The as-printed optical density and
print-through for the patches depicted in FIG. 10A are also shown for
comparison. The test results show that using a high-gel, high-wax cyan
ink formulation, has the effect of preventing ink penetration into the
paper while still enabling some degree of leveling to occur. Some degree
of leveling occurs as judged by the increase in optical density over the
as-printed sample for the irradiated samples with throughput speeds in
the process window of about 500 mm/s to 1000 mm/s. All samples exhibit
acceptable print-through as judged by visual appearance of the back side
images of the 600×600 dpi areas except for FIG. 10F. This is
consistent with the plot in FIG. 11, where the print-through rises above
the acceptable level, PT 0.025, for the slowest throughput speed of 125
mm/s.

[0078] In embodiments of the methods of leveling ink on substrates, it is
desirable to produce leveling of the ink on a substrate surface
substantially without any simultaneous curing of the ink. Curing will
impede leveling of the corrugated structure formed by ink droplet
freezing on substrate impingement. If leveling is impeded, then
micro-banding will not be effectively mitigated and completely missing
lines will not be effectively covered. Curing of the ink results when
cross-linking or polymerization reactions occur in the ink. In
embodiments, the radiation source used for leveling the ink is selected
to emit radiant energy onto the ink that produces substantially no curing
during leveling.

[0079] In other embodiments of the methods of leveling ink on substrates,
a small amount of curing may also occur during the leveling of the ink,
in cases where a portion of the emission spectrum of the radiation source
may be capable of causing curing in the ink composition being leveled,
and this portion is not removed, such as by filtering. For example, this
can occur if the leveling lamp is a deuterium arc lamp with a quartz bulb
(which will pass all UV output), or a cerium doped glass bulb which will
filter UVC (200-290 nm) and UVB (290-320 nm), but will pass UVA (320-400
nm). However, in those embodiments, the radiation source can emit radiant
energy effective to heat the ink to a sufficient temperature to produce
leveling while reducing the ink viscosity at a faster rate and/or by a
larger magnitude, than any cross-linking or polymerization of the ink can
increase the ink viscosity. As a consequence of the ink viscosity being
reduced in this manner by a temperature change, any curing that may occur
in the ink during leveling substantially does not impede leveling and the
desired results of the leveling on the ink can still be achieved.

[0080] In embodiments in which curing of the ink is desired to achieve
robustness of images on substrates, the ink can be exposed to radiant
energy effective to produce the desired curing of the ink composition
subsequent to leveling of the ink.

[0081] It will be appreciated that various ones of the above-disclosed, as
well as other features and functions, or alternatives thereof, may be
desirably combined into many other different systems or applications.
Also, various presently unforeseen or unanticipated alternatives,
modifications, variations or improvements therein may be subsequently
made by those skilled in the art, which are also intended to be
encompassed by the following claims.